Effect of HYP fibers on bulk and surface roughness of wood-free paper

Two typical high-yield pulps (HYPs) used in wood-free paper grades, aspen HYP and birch HYP, were investigated for their effects on the bulk and surface roughness when substituting for hardwood kraft pulp in wood-free fine paper production. Results show that paper bulk, bending stiffness, and opacity can be improved significantly by both aspen and birch HYPs. Birch HYP has greater effects since birch fibers are larger and have higher coarseness. However, increasing HYP substitution rate increases paper surface roughness and decreases paper strength. Selection of HYP species and substitution rate should be made based on the needs of a specific paper grade.